March 07, 2008

Genetic traces of east-to-west human expansion waves in Eurasia

American Journal of Physical Anthropology (early view)

Genetic traces of east-to-west human expansion waves in Eurasia

Raphaëlle Chaix et al.

In this study, we describe the landscape of human demographic expansions in Eurasia using a large continental Y chromosome and mitochondrial DNA dataset. Variation at these two uniparentally-inherited genetic systems retraces expansions that occurred in the past 60 ky, and shows a clear decrease of expansion ages from east to west Eurasia. To investigate the demographic events at the origin of this westward decrease of expansion ages, the estimated divergence ages between Eurasian populations are compared with the estimated expansion ages within each population. Both markers suggest that the demographic expansion diffused from east to west in Eurasia in a demic way, i.e., through migrations of individuals (and not just through diffusion of new technologies), highlighting the prominent role of eastern regions within Eurasia during Palaeolithic times.

"If we extend these results to other parts of Eurasia, which always show proportions of LIC equal to 39% or less, our mtDNA results support a scenario in whichthe expansion diffused westward from the Far East (China and Mongolia) to Europe through recurrent gene flows and/or massive and sudden moves of people."

Indeed, our data point to Central Asia as having playedfirst a role of receiver of genetic diversity coming fromthe Far East, and subsequently, a role of starting pointof expansion waves toward western regions. Such scenariocould explain the current distribution of mtDNAhaplogroups in Eurasia, showing a clear distinctionbetween eastern and western haplogroups and a mixtureof these geographically-distinct haplogroups in CentralAsia (Comas et al., 1998, 2004; Quintana-Murci et al.,2004; Derenko et al., 2007).

No, the "clear distinction between eastern and western haplogroups" is strong evidence against the scenario propounded here. There is simply no way to interpret the mtDNA phylogeographic data as supporting an east-west demic diffusion starting in China.

The basic idea behind the study (looking at "LIC") seems reasonable to me, at least on the surface. But I haven't reviewed in detail all the assumptions underlying it.

The results on the Y chromosome are broadly in line with the phylogeographic evidence:

Regarding Y chromosome data, the proportions of LICobserved between China and western Eurasia were high(76% and above) and such high percentages of LIC arecompatible with the existence of independent expansionevents in China on one hand and in western Eurasia onthe other hand, although the percentage of LIC betweenChina and Central Asia (76%) could also correspond toan extreme case of a diffusion of expansion throughmigrations with an old divergence age between the tworegions (about 1,700 generations), a migration rate nothigher than 0.0005 and a size before expansion N0 equalto at least 500.

Exactly where they go wrong on the mtDNA side, I can't immediately say. The authors themselves mentions one possible confounding factor:

Admixture may be the biggest issue, though the authors downplay the possibility:

This approach has been criticized for being sensitive to acouple of phenomena: (i) the heterogeneity of mutationrates (Aris-Brosou and Excoffier, 1996) and (ii) pastadmixture events, which can both affect the shape of themismatch distributions. However, there is no objectivereason why these processes should affect the easternand western populations differentially. Moreover, oursimulation study comforts us in the validity of ourapproach, in particular since we took into account theheterogeneity of mutations rate in these simulations,this heterogeneity having little impact of the LIC proportions.Finally, the high proportion of observed unimodalmismatch distributions suggests that admixture eventsbetween genetically differentiated populations have beenrare enough not to affect substantially the genetic landscapeof Eurasian populations.

In fact, both the Chinese and Central Asians are most likely products of "admixture events between genetically differentiated populations".

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